The present invention relates generally to an apparatus for and method of producing polymer powders. Specifically, the present invention is directed to an apparatus for and method of producing monodisperse and submicron sized polymer powders from solution.
Polymer powders are crucial to a far reaching assortment of industrial applications. These applications specifically require polymer powders of ever smaller particle size and size dispersity. For example, polymer powders that embody these characteristics are especially useful for forming plastic particles into intricate shapes with precise dimensions. Other applications that would also benefit from these characteristics include polymer alloys, self-reinforced plastic and elastomeric composites, foams, paints and coatings, molecular sieves, membranes, thickeners and thinners of colloidal dispersions, new materials for efficient chromatography and chemical sensors (molecular imprinting), cost extenders for various adhesive formulations, drug delivery systems, free flowing powders to generate more precise objects from 3-D lithography, catalysts containing larger surface area, photoluminescent polymer dots, and artificial enzymes for biomimetics.
Currently, polymer powders are produced by cryogenic grinding of polymer pellets. However, this method severely limits the size and size dispersity of resulting particles. In some cases, even the smallest sized particles with lowest size dispersity produced by grinding methods are inadequate for intended applications. In fact, the best grinding methods known to Applicants produce powders with diameters no smaller than about 100 microns having extremely broad size dispersity. Moreover, grinding methods cannot be used to adequately form microparticles of polyethylene-based waxes.
Gas atomization processes (GAP) overcome several disadvantages inherent in grinding methods by enabling the mass-production of high quality spherical polymers that are not amenable to grinding because of their waxy properties, such as polyethylene glycol. These processes achieve this by using high pressure (approximately 7.6 MNmxe2x88x922) nitrogen gas and a specifically designed nozzle to atomize a molten stream of polymer into fine droplets which cool to form spherical polymer powders. A further description of the process is found in xe2x80x9cAtomize Polymers to Maximize Profit,xe2x80x9d Materials World, vol. 5 no. 7 July (1997), hereby incorporated by reference. This method, however, produces particles no smaller than about 50 microns. Moreover, resulting powder particles are size disperse. Furthermore, GAPs do not work at ambient conditions, since using this method polymers can only be atomized under high pressure and at narrow temperature ranges, for example between about 190-220xc2x0 C. for low molecular weight polymers.
In xe2x80x9cMolecular Dynamics Simulation of Polymer Flow in Nano-Channels,xe2x80x9d Tuzen et al disclose spherical powders between 5 and 200 microns made by a gas atomization process. Poly Preprints, 76, p. 585 (1997). However, resulting powders here are severely limited in terms of size dispersity and are likewise not produced at ambient conditions.
Commercially available vibrating orifice aerosol generators utilize the application of constant pressure and high frequency (10-100 kHz) disturbance of the stream of solution to produce droplets. Typically the droplets so formed have high monodispersity (≈0.1%) but have a lower practical bound on droplet size of about 25 microns. This combined with the high droplet velocity (up to 10 meters per second) and high droplet repetition rate (equal to the nozzle frequency) cause significant difficulties in collecting dry polymer particles from the stream without clustering or coagulation of wet or semi-dry particles.
Clearly, there is room for improvement in the art.
Therefore, it is an object of the present invention to provide a method of and apparatus for producing submicron sized polymer powders.
It is a further object of the present invention to provide a method of and apparatus for producing polymer powders having a size dispersity of as low as about 0.1%.
It is a further object of the present invention to provide a method of and apparatus for producing a wide range of polymer powder sizes the variation of which can easily be controlled by simply altering feed polymer solution concentration and/or apparatus dimensions.
It is a further object of the present invention to provide a method of and apparatus for producing polymer powders at ambient conditions.
These and other objects are achieved in one aspect of the present invention by a method of producing polymer powders. The method comprises the steps of: providing a solution comprising one or more of a wide range of solid materials in a compatible solvent or blend thereof; and applying one or more pressure pulses to a containment vessel containing the solution with a piezoelectric transducer, all at ambient conditions. This results in the formation of aerosol drops which dry to form uniquely small and monodisperse powders.
These and other objects are achieved in a second aspect of the present invention by an apparatus for producing polymer powders. The apparatus comprises: a droplet generating component having a containment vessel for solution having a small orifice. The vessel is connected to a piezoelectric transducer which applies intermittent pressure pulses to the solution at a full range of pressures and temperatures including ambient conditions, thereby causing the ejection of very fine droplets from the vessel. The apparatus may be set to produce droplets resulting in powders having a wide range sizes, some even being monodisperse submicron particles.
These and other objects are achieved in a third aspect of the present invention by a composition comprising polymer powders. These powders have the characteristics of having arbitrary size ranging from about 50 nm to about 50 microns. They also have size dispersity between about 0.1% and 2%.